How do i know if a compound is covalent or ionic? aluminiumoxid is an ionic compound. methan is a covalent compound. how do i know this? i like to have a general explanation. in advance.

Well, aluminum oxide is an ionic compound as it contains elements categorized as being metal and nonmetal, aluminum is metal and oxygen is nonmetal.

Methane is a covalent compound as it contains elements that are categorized as nonmetals. Hydrogen and carbon.
Also if you took the electronegative values of each element in methane and subtracted the larger one from the smaller one, it would result in a value that is found within a particular range of electronegativity differences that represents it's a covalent, more specifically it can help distinguish between polar covalent and or Nonpolar covalent compounds.

Mass is the amount of matter in an object.  weight is the measurement of the pull of gravity on an object.  say for example, you travel to the moon which has a different gravitational field compared to the gravitational field of earth. (gravitational fields: earth = 1, moon = 0.165)  now when you are on the moon, the pull of gravity on you is different than on the earth. and as weight is the measurement of the pull of gravity on an object, that means yes, your weight can and will change on the moon.  however, on the moon, on earth or even on jupiter, the amount of mass inside of you remains the same. meaning your physical mass remains constant.  in conclusion it is possible for your weight to change as your mass remains constant. 

No. an alpha particle is a pair of protons and a pair of neutrons, i.e. a helium